1. Field of the Invention
The present invention relates to directional couplers, and more particularly to directional couplers used in, for example, wireless communication apparatuses that perform communication using high-frequency signals.
2. Description of the Related Art
Known examples of existing directional couplers include a directional coupler disclosed in Japanese Unexamined Patent Application Publication No. 8-237012. This directional coupler is formed by stacking a plurality of dielectric layers on which substantially coil-shaped conductors and ground conductors have been formed. Two of the substantially coil-shaped conductors are provided, one forming a main line and the other forming a sub line. The main line and the sub line are electromagnetically coupled to each other. The substantially coil-shaped conductor is sandwiched between the ground conductors in the stacking direction. A ground potential is applied to the ground conductors. In the above-described directional coupler, when a signal is input to the main line, a signal having a power proportional to the power of the input signal is output from the sub line.
However, in the directional coupler disclosed in Japanese Unexamined Patent Application Publication No. 8-237012, the degree of coupling between the main line and sub line is increased when the frequency of a signal input to the main line is increased (i.e., the amplitude characteristic of a coupling signal is not flat). Hence, even when a signal with constant power is input to the main line, the power of a signal output from the sub line varies when the frequency of the signal varies. Accordingly, an IC connected to the sub line needs to have a capability of compensating the power of the signal in accordance with the frequency of the signal.